You can use dope on the unpainted ones, like Coverite, fabric covering does not sag if properly installed like the 'Kotes, as fabric is able to breath and let trapped air out, you must always be careful of which way the grain is running when installing polyester coverings, it should run spanwise and along the fuselage length for optimum shrinkage, I've used these coverings many, many times as an example, the Chipmunk below is about 10 or more years old and the Partenavia is about 6, and never have I had to use a heat gun to tighten the covering, both were covered with Coverite and doped, then painted with automotive paints.